1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to a configuration for preventing a transfer portion from distorting an image on a sheet.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a copying machine, a printer, and a facsimile machine forms a toner image on an image bearing member such as a photosensitive drum and a transfer belt, transfers the toner image to a sheet conveyed to a transfer portion, and then fixes the image on the sheet using a fixing portion. The image forming apparatus is provided with a correction portion that detects the position or skew of a sheet before conveyed to the transfer portion and optimally corrects relative positional relation with a toner image.
For example, a correction portion detects skew of the sheet at the end of the sheet, nips and rotates the skewed sheet to correct the sheet skew (see Japanese Patent Laid-open No. 2005-53646). After correcting the sheet skew, the image forming apparatus having the correction portion computes misalignment between the sheet tip end and the toner image tip end due to the rotation and changes the image timing or the sheet conveying speed to align the toner image with the sheet tip end.
The conventional image forming apparatus having the correction portion corrects distortion of a toner image formed on the sheet by detecting the skew of the sheet and the misalignment of horizontal registration position before the sheet enters the transfer portion. However, the transferred image may be distorted depending on a sheet conveying condition in which the sheet is transferred to the transfer portion and passes through it.
The conventional apparatus cannot detect the conveying state of a sheet passing through the transfer portion even though the apparatus can detect the skew of the sheet and the misalignment of horizontal registration position before the sheet enters the transfer portion. As a result, the toner image is distorted. As discussed in Japanese Patent Laid-Open No. H7-33286, for example, a detection unit is provided near the transfer portion to detect the conveying state of a sheet passing through the transfer portion. The detection unit can detect the conveying state of a sheet to prevent a toner image from being distorted.
In general, upstream and downstream sheet conveying units of the transfer portion simultaneously convey the sheet when passing through the transfer portion. In this state, a velocity difference may occur between the conveying units and stretch or loop the sheet. When the sheet is stretched, the sheet or the toner image is greatly damaged. In many cases, the downstream sheet conveying unit is given a much lower sheet conveying velocity than the upstream sheet conveying unit.
When the velocity difference is provided, however, the sheet is looped between the sheet conveying units. The looped sheet hinders a normal detection operation even though the detection unit is provided near the transfer portion in order to detect the sheet conveying state. The loop condition varies with the skew between the sheet conveying units or a sheet conveying velocity. The loop condition also varies with an installation condition of the image forming apparatus because distortion of the entire image forming apparatus affects the skew between the sheet conveying unit. The sheet conveying velocity of the sheet conveying unit varies with a period of use extended by the wear of the conveying roller, for example. The loop condition chronologically varies with the installation state or the period of use of the image forming apparatus. It is difficult to detect the sheet conveying state at the transfer portion in anticipation of the loop effect.
The present invention has been accomplished in view of such circumstances. The invention provides an image forming apparatus capable of preventing a transfer portion from distorting a toner image on a sheet.